The present invention relates to a continuously-variable transmission for a vehicle, which includes a belt-type continuously-variable transmitting mechanism and an auxiliary transmitting mechanism disposed in series with the belt-type continuously-variable transmitting mechanism.
Japanese Patent Application Publication No. 60-037455 (corresponding to UK Patent Application Publication No. 2144814) discloses a previously-proposed continuously-variable transmission for a vehicle. In this technique, an auxiliary transmitting mechanism that can attain two shift steps (two speed ratios) for forward running of vehicle is provided in series with a belt-type continuously-variable transmitting mechanism, and the shift step of this auxiliary transmitting mechanism is changed according to a running state of vehicle. Thereby, obtainable range of speed ratio (i.e., obtainable speed-ratio width) of whole of the continuously-variable transmission is made larger without enlarging the size of belt-type continuously-variable transmitting mechanism.
Japanese Patent Application Publication No. 5-079554 discloses another previously-proposed continuously-variable transmission for a vehicle, which includes an auxiliary transmitting mechanism. In this technique, when a shift step (speed ratio) of the auxiliary transmitting mechanism is changed, a cooperative shift for varying a speed ratio of belt-type continuously-variable transmitting mechanism in synchronization with the shift of the auxiliary transmitting mechanism is performed so as to maintain a speed ratio of whole of the continuously-variable transmission (hereinafter referred to as “through speed ratio”) at a constant level. According to the cooperative shift disclosed in this Japanese Patent Application Publication, rotational-speed variations of an engine and a torque converter at the time of shift of auxiliary transmitting mechanism are suppressed because to the through speed ratio is maintained at a constant level between before and after the cooperative shift. Accordingly, a shift shock which is caused due to inertia torques of the engine and torque converter is suppressed.